The yeast, Saccharomyces cerevisiae, offers a good system for the study of eukaryotic DNA replication. Its chromosomal DNA replication is similar to that in higher eukaryotes in that initiation sites are internal and chromosomes contain more than one replication unit. In contrast to the situation in higher eukaryotes, its chromosomal DNA is small enough to be isolated and examined by physical techniques without shearing, and DNA replication mutants are available. Experiments are outlined involving electron microscopic observation of intact, replicating DNA molecules which will provide a replication map of several yeast chromosomes to answer the questions of whether initiation sites are specific, and whether the temporal sequence of replication is constant. Other experiments will determine whether nuclear division mutants are blocked in DNA replication. In addition, the time of synthesis of mitochondrial DNA will be determined and also whether nuclear and mitochondrial DNA share common replication machinery. BIBLIOGRAPHIC REFERENCES: Gubbins, E.J., C.S. Newlon, and J.E. Donelson (1976) Sequence Organization and Expression of a 2 Micron Circular DNA Found in Yeast. Fed. Proc. 35: 1517. Newlon, C.S., R.D. Ludescher, and S. K. Walter (1976) Production of Petites by Mutants of Saccharomyces cerevisiae Defective in DNA Replication. Genetics 83: S55.